张北强余震前震源机制解的一致性特征

Consistency of the focal mechanisms before some strong aftershocks in Zhangbei earthquake series

取层状介质中一点源位错模型,由计算综合地震图得到 P、 S各自最大振幅,通过其最大振幅比值与观测资料拟合的方式来确定震源机制。我们系统地查阅了北京台网、河北台网的数字化记录并补充内蒙古的兴和临时台、集宁台和宝昌台1998年～1999年3月的记录,读取垂直向的 P、 S最大振幅数据,经反演计算得到145个小地震的震源机制解。这些解的优势取向和华北构造应力场的方向相同。定义各个震源机制解的P、N、T轴和构造应力场的3个应力主轴在三维空间的夹角之和为一致性参数,发现在1999年发生的强余震前一致性参数出现异常变化,我们认为这是具有预测意义的现象。

Based on point source dislocation model in layer medium, the focal mechanisms were determined by fitting the observed data with the biggest amplitude ratio of P and S waves calculated from synthetic seismogram. In inversion the digital data in Beijing network, Hebei network, and some other data observed by Xinghe, Jining and Baochang Seismostation in Inner Mongolia were used, and 145 focal mechanisms of small earthquake are inversed with the vertical maximum amplitude of P and S wave. The dominant orientation of the results is consistent with the direction of tectonic stress field in North China. It is significant that there was some abnormal change before the strong aftershock of 1999 in the consistent parameter, which is the sum of the angles between P, N, T axis in focal mechanisms and the corresponding axis in tectonic stress field.